This invention relates to furnace control apparatus and circuitry, and is more particularly concerned with an emergency heat circuit for a microprocessor controlled furnace to provide emergency control of furnace operations for heat in the event of a system failure, e.g., microprocessor failure.
Modern furnaces for the home or for many commercial applications operate at two or more burner rates and several blower speeds, and the operation of these can be sequenced by means of a microprocessor-based control. This is especially true for high-efficiency condenser furnaces of the type described, e.g., in the U.S. Pat. No. 4,729,207, granted Mar. 8, 1988. In these furnaces the level of air flow through or over its heat exchanger is closely controlled; consequently high and low pressure switches are monitored to control inducer and blower motor speed. Burner operation is also closely monitored. The microprocessor of the control circuit contains a suitable algorithm to control the furnace operation for optimum efficiency. However, in the event of microprocessor failure, the furnace should be able to operate at a single speed when the room thermostat calls for heat. Preferably, this should be automatic, without intervention by the home owner, who presumably is unskilled in the workings of the furnace control circuit.
The control circuit should also provide for emergency bypass operation in the event of failure of elements other than the microprocessor, for example, failure of low pressure or temperature sensors, etc.
An emergency heat bypass arrangement for a furnace has been proposed in U.S. Pat. No. 4,723,703. The circuit of that patent provides an alternate path through a relay subject to an emergency heat switch that controls its solenoid. However, its relay is subject itself to malfunction, and it was desired to replace the mechanical relay with opto-isolation or other solid-state switches. It was also desired to provide automatic emergency heat actuation in the event of microprocessor failure.